1. Technical Field
The present invention relates to a method and apparatus for data processing in general, and in particular to a method and apparatus for data detection. Still more particularly, the present invention relates to a method and apparatus for performing digital detection of data stored on an optical medium.
2. Description of the Prior Art
Binary data are typically encoded and stored on optical media as a series of optical modulations. A retrieval of these stored data requires a synchronous clock from a self-clocking analog signal waveform and a detect-and-decode system in a read channel to reconstruct the original binary data. An important element within the read channel is a waveform detector. The waveform detector may be a transition detector (commonly known as a zero-crossing detector) or a peak detector. Generally speaking, transition detectors are utilized for detecting data in Pulse-Width Modulation (PWM) recordings and peak detectors are utilized for detecting data in Pulse-Position Modulation (PPM) recordings.
Digital waveform detectors typically rely on discrete-signal embodiments of well-known analog detection techniques. In essence, an analog signal waveform is initially sampled and then digitized. The digitized samples are subsequently processed digitally to attenuate any unwanted frequency components in order to reconstruct the synchronous clock and data.
One of the more commonly known digital waveform detectors for the detection of data stored on optical disks is called an asynchronous digital waveform detector. The asynchronous digital waveform detector utilizes a straight-line approximation to estimate the location of a detection event--either a peak for PPM recordings or a transition for PWM recordings. This method works well as long as the sample rate is greater than five samples per sinusoid, where a sinusoid describes the highest frequency tone that can be written on optical storage media. Although this sample rate is not considered to be very high, there are at least two reasons why it is desirable to lower the sample rate even further. First, the design of digital circuits will be easier and the power consumption of the entire digital module will be lower if the speed at which the digital logic is required to operate is decreased. Second, under some circumstances, the number of synthesizers can be reduced from two to one, making the digital module count lower and the electronics footprint smaller, which translates to a lower total cost. As the sample rate becomes lower, however, the estimated location of a detection event obtained by straight-line approximation becomes less accurate. For example, as shown in FIG. 9a, an estimate sample at location 93 obtained by a straight-line approximation of actual received samples 91 and 92, does not coincide with actual peak location 94 of an analog signal waveform 95 from a PPM recording. Consequently, it would be desirable to provide an improved method for performing digital detection for data stored on an optical medium at a relatively low sampling rate, without sacrificing detection accuracy.